In my post on design
versus extreme conditions, I pointed how how failing to
consider the extreme operating conditions in addition to the design
conditions could lead to a few surprises. Similarly, considering
what happens for all of the hours between the summer and winter
design conditions is also on the path to successful operation.
Consider the system illustrated below.

The system is located in St. Louis, Missouri. Key features
of the system are as follows:

The system is aVariable Air Volume (VAV) reheat
system serving a multi-use facility on a college
campus and may need to handle up to 40% minimum outdoor air if
there is a major event occurring with a high occupant load.

The system has not been updated to incorporate a
demand controlled ventilation cycle, thus the minimum
outdoor air flow rate is fixed, irrespective of the actual occupant
count. (If you were a retrocommissioning provider scoping the
facility, this would be an important finding and target for
improvement.)

Given the St. Louis climate, the large
minimum outdoor air rate translates into a
requirement for a preheat coil. The preheat coil is
arranged to handle subfreezing air and protects the elements
downstream in the system from freezing.

A freezestat located downstream of the preheat
coil provides an added measure of protection and
locks the system out if the preheat coil leaving air temperature
drops below 38°F.

Given the internal gains in the
facility, the system needs to provide cooling on a
year round basis. Cooling is provided by an integrated
economizer cycle, which uses outdoor air for “free cooling”
until it can no longer meet the discharge temperature set point
requirement, then supplements the outdoor air cooling with chilled
water cooling.

To minimize the need for cooling demand and
reheat, the system’s discharge temperature is
reset between 51 and 58°F depending on humidity levels in a
critical zone and reheat valve position.

On days when it would take more energy to cool the
outdoor air than the return air stream, the
system reverts to minimum outdoor air and uses chilled
water to meet all of the cooling requirements.

The system is also equipped with amixed air low limit cycle to allow it
to start up on a cold morning with out nuisance freezestat
trips. The cycle over-rides the discharge temperature control
signal an limits mixed air temperature to no lower
than 45°F.

Now consider what this system has to do to operate on
Tuesday, January 25, 2005, the day illustrated in the graph
below.

For now, I’ll leave that for you to contemplate. In subsequent
posts, I’ll explore some of the challenges presented by the weather
that occurred on this particular day, which are formidable, despite
the fact that the system never sees one hour anywhere near the
cooling or heating design condition.